Amplifier system



May 12, 1942. c, s, ROOT 2,282,382

AMPLIFIER SYSTEM Filed Aug 19, 1939 a ,14 fi Fig.2. 3+ :44, 5 II:

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CONTACT Low 0N. VOLUME CONTROL RESISTANCE CONTACT AT TOP VOLTS on VOICEcon. in a O O vours on volcscou. g 5 O 100 400 3,000 ,000 10,000 so90100 400 4000 4500 mono Fig.5.

CONTACT AT TAP 26 VoLTS 0N VOICE con. S c O Inventor: Charles 5. Root,

y H is Attorney.

the amplifier in degenerative phase.

Patented May 12, v1942 OFFICE AMPLIFIER SYSTEM Charles s. Root, stramra,com. assignor to General Electric Company, a corporation of New YorkApplication August 19, 1939, Serial No. 291,032

Claims.

My invention relates to amplifiers and more particularly to suchamplifiers as are used in radio receivers for household reception, forexample.

It has to do with degeneration circuits as used in such amplifiers andhas for one of its objects to effect certain improvements therein withrespect to the response characteristic of the amplifiers at lowfrequencies.

One of the objects of my invention is to provide means whereby theresponse of the amplifier may be reduced by degeneration to a very lowvalue at low frequencies thereby to reduce certain undesired phenomenawhich occur chiefly at frequencies below two hundred cycles.

Commonly, in radio receivers, a condenser is connected in shunt with theaudio signal channel usually in series with a resistance, for what isknown as bass compensation. This condenser medium and high frequencycurrents through the system and thus accentuates the reproduction of lowfrequency currents. This produces a fuller and more pleasing tone fromthe output of the loudspeaker.

In accordance with my invention a switch is arranged to connect afeed-back voltage from the output of the amplifier across said condenserand this voltage is impressed upon the input to The condenser is sochosen that at high frequencies this potential is of inappreciable valuebut at very low frequencies it is large enough very greatly to reducethe response of the amplifier. In this way, the tendency of theloudspeaker to overload, for example, during reception of strong bassprograms is reduced, as is also the response of the receiver tosixty-cycle hum currents which may be present as modulations on thereceived carrier wave. It also has the effect of reducing booming soundsin response to voices having a strong bass resonance, as well as toreduce the tendency of the receiver to howl or produce fluttering soundsdue to acoustic feed-back between the loudspeaker and oscillator, suchas occurs frequently during short wave reception.

A further object of my invention is to combine such a feed-back systemwith a system of the type described in my patent application Serial No.221,319, filed July 26, 1938, entitled Feed-back circuit and which isassigned to the same assignee as my present application. Thisapplication is a continuation in part of that application. In this waycertain additional effects are produced which will later be explained.

The novel features which I believe to be characteristic of my inventionare set forth with particularity in the appended claims. My inventionitself, however, both as to its organization and method of operationtogether with further objects and advantages thereof, may best beunderstood by reference to the following description taken in connectionwith the accompanying drawing in which Fig. 1 represents an embodimentof my invention; Fig. 2 represents a modification thereof and Figs. 3,4, and 5 represent certain characteristics relative to its operation.

Referring to Fig. 1 of the drawingI have shown therein, at I a diode,which may comprise the diode detector of an ordinary radio receiver.This diode is connected in series with a resistance 2 and the secondarywinding of a radio frequency transformer 3. This transformer maycomprise the last intermediate frequency transformer of the receiver, ifthe receiver be one of the superheterodyne type, or may comprise atransformer to which currents of the frequency received upon the aerialare applied, if the receiver be one of the tuned radio frequency type.The resistance 2 is the ordinary diode load resistance and is shunted bya condenser 21 having a-value suitable to by-pass currents of radiofrequency and to give the combination of condenser 21 and resistance 2 asuitable time constant for production upon resistance 2 of aunidirectional potential having the desired audio signal modulationstherein. These signal modulations are applied through a couplingcondenser 4 to the diagonally opposite'terminals 5 and 6 of a Wheatstonebridge having arms I, 8, 9 and Ill. The arm III of this bridge maycomprise the usual volume potentiometer of a radio receiver having avariable tap I I to which the control electrode of the first dischargedevice I2 is connected. The output from this discharge device I2 issupplied through a coupling condenser I3 to the input of a secondelectron discharge device I4 from which the output currents are suppliedthrough an audio frequency transformer l5 across the voice coal l6 of aloudspeaker.

Anode operating potential is impressedupon theterminals l1 and suppliedthrough a primary winding of transformer I5 to the anode of dischargedevice I4 and through resistance III to the anode of discharge devicel2. Resistance IB-is the usual grid resistance connected between thegrid of discharge device I4 and ground, and resistance 20 is the usualbias resistance connected between the cathode of discharge device I4 andground, this resistance being short circulted for signal frequencycurrents by condenser 2|.

Potential from the voice coil I6 of the loudspeaker is supplied throughconductors 22 and 23 to a second pair of diagonally opposite corners ofthe Wheatstone bridge. The bridge is, of course, so balanced withrespect to these currents, that the points 5 and 6 are at equalpotential with respect to these feed-back currents and, of course, theconductors 22 and 23 are at equal potential with respect to currentssupplied from the detector to the bridge.

' voice coil is supplied to the grid of amplifier 12.

Hence there is no feed-back either degenerative or regenerative throughthis path which includes conductors '22 and 23. As the contact H ismoved downward however, potential from conductor 22 is supplied to thegrid, this potential increasing throughout the length of resistance l0.Conductors 22 and 23 are connected properly to the loudspeaker voicecoil to cause this potential to be degenerative. Thus when thpotentiometer is adjusted near its lower terminals as would be the caseduring reception of strong stations, the amplifier is. highlydegenerative thereby to' reduce reproduction of hum currents,distortion, etc., generated in the amplifier itself, whereas, when thepotentiometer is near the upper end of the volume control resistance l0,as would be the case during the reception of a very weak station, thisdegeneration is removed thereby increasing the sensitivity of theamplifier.

The bridge may be so balancedthat this degeneration is removed beforethe contact ll reaches the upper end of the volume control and so thatwhen the contact is at the upper-extremity of the volume controlresistance III, the amplifier becomes regenerative thereby additionallyincreasing the sensitivity of the amplifier by reason of regeneration. I

Across the lower portion of potentiometer I0 is connected a shunt pathcomprising resistance 24 and condenser 25. This condenser is the usualbass compensation condenser, which is commonly used in radio receiversto by-pass, to some extent, the high and medium frequency currentsthereby to accentuate the reproduction in the output circuit of the lowfrequency currents relative to the high and medium'frequency currents.This produces a somewhat more full and more pleasing tonal response fromthe loudspeaker. This increased response of the receiver to lowfrequency.

accordance with my invention, by supplying a feed-back voltage from theoutput circuit oithe amplifier l2 through resistance 2'! acrosscondenser 25. This potential, which is thus supplied to condenser 25, isimpressed upon the potentiometer I0 through resistance 24 and hence uponthe grid ofthe amplifier in degenerative phase. Condenser 25 may haveacapacity of about .002 microfarad so that at sixty'cycles its impedanceis about. 1.5 megohmswhereas at 600 cycles its impedance is only 150,000ohms. Thus this condenser effectively short circuits the feedbackpotential supplied through resistance 21 at higher frequencies abovefour or five hundred cycles, whereas at lower frequencies this potentialis appreciable and increases with lower frequencies, therebyincreasingly reducing the response of the amplifier at the lowerfrequencies.

The connection of resistance 21 as shown not only reduces the responseat low frequencies by the production of degeneration but it has thefurther effect of partially shunting condenser 25.-

internal impedance of source 'of operating voltage connected betweenterminals l'l. Resistance 21 may be of the order of 680,000 ohms,resistance IQ of the order of 470,000 ohms, resistance l8 of small.These impedances thus comprise an efcurrents is highly desirable withrespect to tonal 'in such receivers to provide a switch such as thatfective shunt at low'frequencies across the condenser 25, whosereactance at cycles may be -of the order of 1,500,000 ohms and thusreduce its eflect of accentuating low frequency currents just as wouldbe the case were the condenser short circuited by the closing of switch26 but to a lesser extent.

The portion of the degenerative voltage on condenser 25 which issupplied to the grid of amplifier I2 is of course greatest when contactII is exactly upon the point 28 to which resistance 24 is connected.This portion, however, is reduced upon movement of the contact II ineither direction. Thus upon movement of the contact ll from.the point 28downward the degenerative voltage supplied through resistance 21 to thegrid of discharge device I2 is reduced at the same time that thedegenerative voltage supplied through conductor 22 to the grid ofdischarge device I2 is increased. The reduction of the indicated at '26whereby. this condenser may be erative voltages to be supplied throughconductor 22.

It is desirable, however, to take advantage of the improved tonalresponse produced by the condenser 25 and at the same time to arrange toavoid all of the undesirable effects mentioned above, by providing aswitch for reducing bass response in a different manner. It has beenfound that this result may largely be secured. in

age produced by conductor 22 may be entirely removed when the contact His at the upper 2 may be of 330,000 ohms and condenser 4 of .005 v pointof resistance I0. The degenerative voltage ml. or 600,000 ohms at sixtycycles whereas the upper portion of resistance l0 may be of one megohm.This means that when contact II is at the upper end of resistance l9approximately half of the voltage supplied through resistances 21 and 24to the potentiometer is still impressed senting this relationship whenswitch 29 i open.

Switch 26, ordinarily is not used in the system and is shown on thedrawing only by way of illustrating the means for removing the basscompensation over which my present method is an improvement. Curve B ofthis same figure represents the frequency response characteristic withswitch 29 closed. It will be seen that the response of the amplifier isvery much reduced at frequencies below 200 cycles, by reason of thisdegeneration.

Fig. 4 shows corresponding curves with contact ll about one-fifth of theway up from the bottom of the potentiometer III, the lower portion ofthe potentiometer being also of one megohm. This would be the positioncorresponding to a low level of speaker output. It will be seen thateven with'switch 29 in the closed position with degeneration applied theamplifier has a fairly large response even at so low a frequency asninety or one hundred cycles. Thus switch 29 effects a noticeablereduction of low frequencies, but not enough to make the tone sound toothin as might occur because the ear does not hear low frequencies aswell at low levels as at higher levels.

Fig. shows curves A and B corresponding with curves A and B of Figs. 3and 4 but with the contact I lpositioned at the point 28 which is themidpoint on the resistance M. In addition, this figure shows curve Cwhich indicates the response of the amplifier with switch 29 open andcondenser 25 shorted by means of a'switch such as that indicated at 26so that the condenser 25 is not effective to accentuate the lowfrequencies this being a very commonly employed means of removing thebass compensation. It will be seen that the accentuation of lowfrequency currents is somewhat reduced by'short-circuiting the condenseras indicated by curve C, but that a much greater reduction is attained,particularly at very low frequencies, by applying degeneration to thecondenser through switch 29 instead, as indicated by curve B.

Fig. 2 represents a slight modification in my invention in which thepurpose of switches .29 and 26 is served by the single switch element30. In the position shown the degeneration from the output of theamplifier is supplied through resistance l9 and switch 39 to oneterminal of the condenser. With the switch moved downward the broadenedend 3| thereof bridges contacts 32 and 33 and thereby short circuits thecondenser while resistance I9 is simultaneously connected to groundthrough resistance 9. The resistance I9 thus serves the purpose of theusual grid resistor in the grid circuit of discharge device 34. Whenswitch 39 is moved to its upper position the short circuit is removedand this degeneration through resistance l9 and condenser 25 is alsoremoved, the lower terminal of resistance l9 being grounded-at 35 by theswitch.

Under this condition low frequency currents are accentuated by the basscompensation condenser 25 in the normal manner.

It will be noted that one advantage and difference of Fig. 2 over Fig. 1is that it eliminates one resistor, the resistor 21 of Fig. 1. In Fig.2, however, the total DC resistance from the grid of discharge device3km ground, embodying resistors I9, 24, the lower half'of potentiometerl0 below the tap 28 and resistor 9, may be too large for satisfactoryoperation of discharge device 34.

In this figure the discharge device 34 is indicated as one of the beamtype of output tubes now commonly employed in which electrodes 36 serveas focusing members for the beam of electrons which impinges upon theplate.

While in the above discussion of the invention I have mentionedparticular values of the various circuit components, it will beunderstood that such values are subject to very wide variation and theyare indicated only by way of a single example of such values as may beused. They are not intended as any limitation upon the present inventionsince any of these values may be varied through extreme ranges.

While I have shown particular embodiments of my invention it will beunderstood that I do not wish to be limited thereto since variousmodifications both in the circuit arrangement and in theinstrumentalities employed may. be made.

without departing from the spirit and scope of my invention as set forthin the appended claims.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. In combination, an amplifier, two feed-back paths from the output tothe input of said amplifier, and means to vary the amplification of saidamplifier and simultaneously to vary the effectiveness of said twopaths, the effectiveness of one of said paths being varied by said meansto a greater extent than the other.

2. In combination, a signal channel including an amplifier arranged toamplify currents having frequencies extending over a wide range, aresistance in shunt with said channel, a condenser, means to connectsaid condenser in a path in shunt to said channel to bypass highfrequency currents in said range, thereby to accentuate low frequencycurrents, and means to connect said resistance in a path in shunt withsaid condenser thereby to reduce said accentuation, said amplifier beingincluded in said channel between said condenser and resistance.

3. In combination, a signal channel including an amplifier arranged toamplify currents having frequencies extending over a wide range, aresistance in shunt with said channel, av condenser, means to connectsaid condenser in a path in shunt to said channel to bypass highfrequency currents in said range, thereby to accentuate low frequencycurrents," means to connect said resistance in a path in shunt with saidcondenser thereby to reduce said accentuation,

said amplifier being included in said channel between said condenser andresistance, and means to supply the voltage impressed on said condenserfrom said resistance through said connecting means to the input of saidamplifier in degenerative phase whereby said accentuation of lowfrequency currents is reduced both by the short circuit effect of saidresistance on said con denser and by the degeneration produced by saidconnection.

4. In combination, an audio amplifier, abridge adapted to have audiofrequency currents to be amplified 1 supplied across diagonally oppositepoints thereof and having feedback currents from the output of saidamplifier supplied across other diagonally opposite points thereof, theinput to said amplifier being connected between one of said firstmentioned points and avariable point on that oneof the two opposite armswhich arm to said intermediate point whereas feed-' back currentssupplied to the input due to said is on the side .of the bridge fromwhich said feed- Y bridge are increased from one end ofthe arm to I theother.

5. In combination, an amplifier, two feedback paths from the output tothe input of said amplifier arranged to produce degeneration, meansadjustable over a range to vary the volume of signal supplied to saidamplifier for amplification thereby, and simultaneously to vary theeffectiveness of both of said paths, and means in said paths to producegreater response'of said.

amplifier at frequencies below a frequency intermediate in the range offrequencies amplified by said amplifier when said means is adjustedtosupply low volume of signal to saidamplifier and to produce lowerresponse at frequencies below v said intermediate frequency than isproduced at said intermediate frequency when said means is adjusted tosupply a high volume of signals to saidamplifier.

6. In combination, an amplifier, two feedback;

paths from the output to the input of said amplifier arranged to producedegeneration, means adjustable over a range to vary the volume of signalsupplied to said amplifier for amplification thereby, and simultaneouslyto vary the.

effectiveness of both of said paths, said means being effective toreduce the effectiveness of both of said paths when adjusted from anintermediatepoint of said range to increase the volume and said meansbeing effective to reduce the effectiveness of one path and increasethat of the other as said means is adjusted from said intermediate pointto decrease the volume, and means included in said paths to proportionthe voltage supplied therethrough to render the response of saidamplifier in the neighborhood of sixty to one hundred cycles less thanthat in the range between four hundred and a thousand cycles when saidmeans is adjusted at said intermediate point and at points to supplylarger volume of signals to said amplifier and to render the response inthe neighborhood of sixty to one hundred cycles greater than thatbetween four hundred and a thousand cycles when said means is adjustedfor low volume.

7. ,The combination, in an audio amplifier, of a bridge having signalvoltage supplied across one diagonal thereof, means to supply signalvoltage from the output of said amplifier across another diagonalthereof, the input of said amplifier being connected between a fixedpoint of said bridge and a point on an arm of said bridge suchithatsaidvoltage from the output is supplied to said amplifier indegenerative phase, said point being variable along said arm for volumecontrol, a resistance connected between the output of said amplifier anda point of said arm intermediate the range of variation of said variablepoint to supply additional degenerative voltage to said amplifier, and abase compensation condenser connected between the terminal of saidresistance remote from said arm and the low signal end of said arm, saidcondenser being so large that said additional degenerative voltage issubstantially ineifective'to reduce the amplification of said amplifierat frequencies above four hundred cycles except when said variable pointis in the region of said point on said arm to which said resistance isconnected.

8. The combination, in an audio amplifier, of Y a bridge having signalvoltage supplied across one diagonal thereof, means to supply signalvoltage from the output of said amplifier across another diagonalthereof, the input of said amplifier being connected between a fixedpoint of said bridge and a point on an arm of said bridge such that saidvoltage from the output is supplied to said amplifier in degenerativephase, said point being variable along said arm for volume control, aresistance connected between the output of said amplifier and a point ofsaidarm intermediate the range of variation of said variable point tosupply additional degenerative voltage to said amplifier, and a basecompensation condenser connected between the terminal of said resistanceremote from said arm and the low signal end of said arm, said condenserbeing so large that said additional degenerative voltage issubstantially inefiective to reduce the amplification of said amplifierat frequencies above one thousand cycles.

9. The combination in a voice current amplifier, of a volume controlpotentiometer, the input of said amplifier being connected between a lowpotential terminal and a variable point of said potentiometer, aresistance and a condenser connected across a low potential portion ofsaid potentiometer, means to supply voltage from the output of saidamplifier across said condenser and thence through said resistance tosaid po-- tentiometer to produce degeneration of said amplifier, saidcondenser and resistance being so proportioned that the output of saidamplifier per unit of input in the region of sixty cycles is less thanthat in the region of two hundred and fifty cycles for all high volumeadjustments of said variable'po'int.

10. The combination, in a voice current amplifier, of a volume controlpotentiometer, the input of said amplifier being connected between alow.

, potential terminal and a variable point of said potentiometer, aresistance and a condenser connected across a low potential portion ofsaidpotentiometer, means to supply voltage from the output of saidamplifier acrosssaid condenser and thence through said resistance tosaid potiometer to produce degeneration of said amplifier, saidcondenser and resistance being so proportioned that the response of saidamplifier in the region of sixty to one hundred cycles is less than thatin the region of four hundred cycles for all high volume adjustments ofsaid variable point and is higher than that in the region of fourhundred cycles for low volume adjustments of said potentiometer;

- CHARLES S. ROOT.

